FIG. 1 shows an example of the conventional device that employs such a device isolation.
Referring to FIG. 1, the device is a typical HEMT and includes a channel layer 3 of undoped GaAs and an electron supplying layer 5 of n-type AlGaAs that is provided on the channel layer 3 with an undoped spacer layer of AlGaAs interposed therebetween. Further, a cap layer 6 of n-type GaAs is provided on the electron supplying layer 5, and another cap layer 7 that contains layers of n-type AlGaAs and n-type GaAs is provided further on the cap layer 7. On the cap layer 7, a number of gate electrodes 8A, 8B, . . . , are provided in correspondence to a number of devices formed on the substrate.
In order to isolate the individual devices, the device of FIG. 1 uses an isolation region 9 that is formed in correspondence to the boundary between adjacent devices such that the region 9 extends from the surface of the layer 9 toward the substrate 1, passing through the layers 3-7. Typically, the region 9 is formed by incorporating oxygen or chromium ions by an ion implantation process and has an increased resistivity due to the foregoing pinning of the Fermi level. Thereby, the passage of the carriers from one device to another device is prevented.
Further, the foregoing layers 3-7 forming the active part of the device are formed on a semi-insulating GaAs substrate 1 that is covered with a buffer layer 2 such that the buffer layer 2 isolates the active part of the device from various adversary surface states or defects that are formed on the surface of the substrate 1. Typically, the buffer layer 2 is formed of an undoped AlGaAs and has a large resistivity. Thereby, each device is laterally isolated by the device isolation region 9 and vertically by the buffer layer 2, and the penetration of the carriers from one device to an adjacent device is prevented.
In such a conventional isolation structure, there exists a problem in that, although the penetration of the carriers from one device to the adjacent device can be minimized, the shielding of the electric field between the devices cannot be achieved successfully. It should be noted that the high purity buffer layer 2 lacks electric charges therein and passes the line of electric force and hence the electric field freely. Thereby, the electric field of one device penetrates into the region of the adjacent device and the operational characteristic of the device such as the threshold voltage tends to be influenced by the state of the adjacent device. In other words, the conventional device of FIG. 1 cannot eliminate the so-called side gate effect satisfactorily. It will be easily understood that the side gate effect becomes conspicuous with increasing integration density and decreasing separation between the devices.
In order to eliminate the foregoing problem, there is a proposal to form the buffer layer such that the buffer layer includes electric charges.
FIG. 2 shows a conventional process for achieving such an elimination of the side gate effect.
Referring to the drawing showing the case for growing a device on a silicon substrate 1A, a first buffer layer 2C of semi-insulating GaAs layer is provided on the substrate 1A for achieving a lattice matching, and a semi-insulating second buffer layer 2A of AlGaAs is grown on the first buffer layer 2C such that the buffer layer 2A contains oxygen ions therein. As a result of the electric charges incorporated in the layer 2A in the form of oxygen ions, the buffer layer 2A can now interrupt the electric field penetrating from the adjacent devices and the side gate effect is effectively eliminated. Further, the oxygen ions form the deep impurity level in the AlGaAs crystal and the buffer layer 2A shows a high resistivity as a result of the pinning of the Fermi level similarly to the device isolation region 9. On the first buffer layer 2A, the layers 3-7 are grown similarly to the structure shown in FIG. 1.
In the structure of FIG. 2, the buffer layer 2A has to be grown under the existence of oxygen. For example, the buffer layer 2A may be grown by admixing small amount of oxygen or water vapor into the organic source of Al, Ga and As. However, oxygen is a chemically active species and the oxygen molecules thus introduced easily cause reaction with the wall of the reaction vessel or piping systems. Once absorbed, the oxygen molecules tend to stick on the vessel wall even after repeated flushing, and causes a contamination of the essential part of the device such as the channel layer 3. In order to avoid this problem, one may use a different epitaxial apparatuses for fabricating the semiconductor device such that the essential part of the device is grown in an apparatus that is different from the apparatus used for growing the buffer layer 2A. However, the use of such separate apparatuses inevitably necessitates a transportation step of taking out the half product from the reaction vessel of the first apparatus and mount the same on the reaction vessel of the second apparatus for further epitaxial process. During the transportation, it will be easily understood that the surface of the buffer layer 2A may be contaminated. In the device such as HEMT, such a contamination of the buffer layer 2A is unacceptable, as the channel layer 3 that forms the most critical part of the device is grown directly on the buffer layer 2A.
In order to eliminate the foregoing problem of contamination and to obtain a buffer layer of AlGaAs that contains oxygen ions as the impurity, there is a proposal in the Japanese Laid-open Patent Application No. 1-220432 to use an organic molecule that contains oxygen for the source of the group III element during the growth of the buffer layer. According to this proposal, an organic source such as trimethoxy gallium or trimethoxy aluminum is used for the source of oxygen and simultaneously for the source of the group III element. The organic source thus supplied decomposes in the reaction vessel in the vicinity of the substrate and release the group III element and oxygen. The oxygen atoms thus released in turn are combined immediately with Al atoms that are released also in the vicinity of the substrate because of the large affinity between aluminum and oxygen, and Al and O are deposited on the surface of the substrate without causing substantial contamination of the reaction vessel. By supplying oxygen in the form of stable organic molecules, one can avoid the contamination of the piping system as well.
In this conventional process, however, there arises a problem in that one cannot supply sufficient amount of oxygen into the reaction vessel and hence into the buffer layer. It should be noted that the flowrate of the organic source of the group III element determines the growth rate as well as the composition of the epitaxial layer and has to be controlled precisely. Generally, the flowrate of the organic source of the group III elements is substantially smaller than the flowrate of the organic source of the group V element. Thereby, the concentration of the oxygen in the reaction vessel after the decomposition becomes inevitably low. Therefore, this process does not provide the buffer layer having the desired device isolation effect.